transit_relay: add Producer/Consumer flow control
transit_relay: add Producer/Consumer flow control

This limits the buffering to about 10MB (per connection*direction).
Previously, if the sender had more bandwidth than the receiver, the
transit relay would buffer the entire file. With this change, the sender
will be throttled to match the receiver's downstream speed.

warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole